It has been conventional to distill coal tar and produce a fraction of intermediate boiling point (180.degree.-300.degree. C.) and extract from this fraction so-called tar acids, primarily phenols and cresols and some xylenols with aqueous base such as aqueous sodium hydroxide. The raffinate from such extraction contains naphthalene, methylnaphthalene isomers, biphenyl and a variety of nitrogen containing compounds which are collectively referred to as tar bases. Various references describe the extraction of this raffinate (usually after distillation to remove naphthalene and lower boilers and also some higher boilers) with weak acid such as 20% sulfuric acid to produce an organic raffinate containing methylnaphthalene and an aqueous extract which, upon neutralization, forms an organic layer containing the tar bases. Examples of such processes are described in U.S. Pat. No. 2,456,774 to Engel (1948) and page 391 of Kirk & Othmer, Encyclopedia of Chemical Technology, Vol. 11 (1st Edition 1953). U.S. Pat. No. 3,412,168 to Masciantonio (1968) discloses a process of liquid phase extractions with sulfuric acid, caustic solution and water, followed by distillation. It appears that tar acids remain in the material of U.S. Pat. No. 3,412,168 in a significant quantity until the caustic solution extraction.
Indole is a valuable chemical used, for example, in the production of tryptophan and in fragrances. While various reports have been made of the identification of indole in coal tar, an economical process for recovering such indole has not been developed. Specifically, the above processes involving extraction with acid do not produce indole as a significant component in the tar base organic layer generated by neutralization. Instead it generally polymerizes and must be disposed of as a gummy waste material.